1. Field of the Invention
The invention generally relates to the field of refrigeration, and more particularly but not by way of limitation, to a control circuit and method for sequentially defrosting refrigerated display cases.
2. Discussion
Refrigerated display cases located in grocery stores and the like are commonly arranged together in a contiguous line or series of cases. The cases are placed side-by-side and often display foods sharing the same refrigeration demands. In refrigerating the case, a fan circulates cold air in a duct that encircles the case. An evaporator coil of a refrigeration system is located in the duct, so that the circulating air is cooled by the cold refrigerant flowing through the evaporator coil.
Over several hours of operation of the refrigeration system, the air flowing through the duct entrains water vapor from the ambient air. This water vapor condenses and freezes on the cold evaporator coil, decreasing the heat transfer efficiency between the refrigerant in the evaporator coil and the air in the duct. Because of this, the evaporator coil needs to be periodically defrosted. Thus, each case has a refrigeration cycle that includes a defrost cycle of operation. During the refrigeration cycle, the refrigeration system cools the case. During the defrost portion of the refrigeration cycle, or that is, during the defrost cycle, a defrost heater melts condensation that has frozen on the evaporator coil.
Defroster heaters are located in the duct near the evaporator coils. An improved evaporator/defroster unit is described in related U.S. application Ser. No. 09/877,566, entitled xe2x80x9cAn Enclosable Evaporator/Defroster Unit for a Refrigerated Display Case.xe2x80x9d In the related application, the evaporator/defroster unit is positioned in the air duct so that, by closing dampers, an enclosure is formed about the evaporator coil to prevent heat transfer from the enclosure during the defrost cycle. Such dampers are automatically closed by actuators during the defrost cycle. Closing the dampers retards heat transfer from the enclosure by all three modes of heat transfer, namely by convection, conduction, and radiation. This allows operation of the fans that circulate air through the ductwork during the defrost cycle. Thus, an air curtain can be maintained across an access opening for the case during the defrost cycle.
It is now common practice to defrost the evaporator coils in a series of cases at the same time, in part because contiguous refrigerated display cases often share a common defrost timer. It is also common to defrost the evaporator coils every 6-8 hours. There are several notable problems with this approach to defrosting the evaporator coils of several cases at the same time.
One problem is that defroster units of the existing art generate a lot of water vapor during a defrost cycle. If a line of contiguous cases is defrosted at the same time, an undesirable frost build-up occurs within the case.
Another problem caused by defrosting the cases at the same time is the need for greater electrical power at the same time. Because the defroster unit wiring is often on the same circuit for a given series of cases, this in turn causes a need for larger wiring sizes to carry the high current demand required for the defrost cycle. Additionally, because the cost of power from public utilities is often based on peak demands, the cost of power may be greatly increased by defrosting all the cases at the same time.
Yet another problem with defrost control systems of the existing art is that many are highly complex with digital components and programmable controllers. This makes repairs difficult for repairmen of ordinary skill in the refrigeration art, who are often only familiar with non-digital electrical components. The term xe2x80x9cnon-digitalxe2x80x9d refers to relays, contactors, sensors, coils, switches and any other component that generally does not process digital information.
One of the most expensive aspects of the existing practice of defrosting a series of contiguous cases at the same time is that it often leads to food spoilage. By shutting down the refrigeration cycles of contiguous cases at the same time, there can be an increase in the temperature of the food product in the cases. Also, there is often a greater increase in the display section temperature of each case due to the combined effect of defrosting several contiguous cases at the same time.
Thus, there is a need for an improved method and apparatus for defrosting refrigerated display cases that avoid the problems created when refrigerated display cases are simultaneously defrosted and that avoid the problems of having complex digital components.
The present invention provides a method and a defroster control circuit for sequentially defrosting a series of refrigerated display cases. At the beginning of the refrigeration cycle, a refrigeration time is set to zero and an elapsed refrigeration time is subsequently monitored. When the refrigeration time equals an activation time, the defrost cycle for the first case is started. The defrost cycle for the first case terminates based either on a defrost time or temperature criterion.
After the first refrigerated display case is defrosted, the refrigeration cycle restarts for the first case and the evaporator coils in the other refrigerated display cases in the series of refrigerated display cases are sequentially defrosted. When all the refrigerated display cases in the series have been defrosted, the elapsed refrigeration time is reset to zero and the normal refrigeration cycle is resumed for the last case defrosted.